[PATCH v7 1/1] pull: add ff-only option

[Felipe Contreras <felipe.contreras@gmail.com>]

It is very typical for Git newcomers to inadvertently create merges and
worse; inadvertently pushing them. This is one of the reasons many
experienced users prefer to avoid 'git pull', and recommend newcomers to
avoid it as well.

To avoid these problems, and keep 'git pull' useful, it has been
suggested that 'git pull' barfs by default if the merge is
non-fast-forward, which unfortunately would break backwards
compatibility.

This patch leaves everything in place to enable this new mode, but it
only gets enabled if the user specifically configures it;

  pull.rebase = ff-only.

Later on this mode can be enabled by default.

For the full discussion you can read:

https://lore.kernel.org/git/5363BB9F.40102@xiplink.com/

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
---
 Documentation/config/branch.txt |  2 ++
 Documentation/config/pull.txt   |  2 ++
 builtin/pull.c                  |  6 +++++-
 rebase.c                        |  2 ++
 rebase.h                        |  3 ++-
 t/t5520-pull.sh                 | 36 +++++++++++++++++++++++++++++++++
 6 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/Documentation/config/branch.txt b/Documentation/config/branch.txt
index cc5f3249fc..715987c759 100644
--- a/Documentation/config/branch.txt
+++ b/Documentation/config/branch.txt
@@ -81,6 +81,8 @@ branch.<name>.rebase::
 	"git pull" is run. See "pull.rebase" for doing this in a non
 	branch-specific manner.
 +
+When `ff-only` (or just 'f'), the rebase will only succeed if it's fast-forward.
++
 When `merges` (or just 'm'), pass the `--rebase-merges` option to 'git rebase'
 so that the local merge commits are included in the rebase (see
 linkgit:git-rebase[1] for details).
diff --git a/Documentation/config/pull.txt b/Documentation/config/pull.txt
index 5404830609..060bacc63c 100644
--- a/Documentation/config/pull.txt
+++ b/Documentation/config/pull.txt
@@ -14,6 +14,8 @@ pull.rebase::
 	pull" is run. See "branch.<name>.rebase" for setting this on a
 	per-branch basis.
 +
+When `ff-only` (or just 'f'), the rebase will only succeed if it's fast-forward.
++
 When `merges` (or just 'm'), pass the `--rebase-merges` option to 'git rebase'
 so that the local merge commits are included in the rebase (see
 linkgit:git-rebase[1] for details).
diff --git a/builtin/pull.c b/builtin/pull.c
index 17aa63cd35..ba2777c401 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -1033,8 +1033,12 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 			}
 			free_commit_list(list);
 		}
-		if (!ran_ff)
+		if (!ran_ff) {
+			if (opt_rebase == REBASE_FF_ONLY)
+				die(_("The pull was not fast-forward, please either merge or rebase.\n"
+					"If unsure, run 'git pull --merge'."));
 			ret = run_rebase(&curr_head, merge_heads.oid, &rebase_fork_point);
+		}
 
 		if (!ret && (recurse_submodules == RECURSE_SUBMODULES_ON ||
 			     recurse_submodules == RECURSE_SUBMODULES_ON_DEMAND))
diff --git a/rebase.c b/rebase.c
index f8137d859b..eba1253552 100644
--- a/rebase.c
+++ b/rebase.c
@@ -20,6 +20,8 @@ enum rebase_type rebase_parse_value(const char *value)
 		return REBASE_FALSE;
 	else if (v > 0)
 		return REBASE_TRUE;
+	else if (!strcmp(value, "ff-only") || !strcmp(value, "f"))
+		return REBASE_FF_ONLY;
 	else if (!strcmp(value, "preserve") || !strcmp(value, "p"))
 		return REBASE_PRESERVE;
 	else if (!strcmp(value, "merges") || !strcmp(value, "m"))
diff --git a/rebase.h b/rebase.h
index cc723d4748..127febeb7c 100644
--- a/rebase.h
+++ b/rebase.h
@@ -7,7 +7,8 @@ enum rebase_type {
 	REBASE_TRUE,
 	REBASE_PRESERVE,
 	REBASE_MERGES,
-	REBASE_INTERACTIVE
+	REBASE_INTERACTIVE,
+	REBASE_FF_ONLY
 };
 
 enum rebase_type rebase_parse_value(const char *value);
diff --git a/t/t5520-pull.sh b/t/t5520-pull.sh
index 9fae07cdfa..d96bdc90cc 100755
--- a/t/t5520-pull.sh
+++ b/t/t5520-pull.sh
@@ -819,4 +819,40 @@ test_expect_success 'git pull --rebase against local branch' '
 	test_cmp expect file2
 '
 
+test_expect_success 'git pull fast-forward (ff-only)' '
+	test_when_finished "git checkout master && git branch -D other test" &&
+	test_config pull.rebase ff-only &&
+	git checkout -b other master &&
+	>new &&
+	git add new &&
+	git commit -m new &&
+	git checkout -b test -t other &&
+	git reset --hard master &&
+	git pull
+'
+
+test_expect_success 'git pull non-fast-forward (ff-only)' '
+	test_when_finished "git checkout master && git branch -D other test" &&
+	test_config pull.rebase ff-only &&
+	git checkout -b other master^ &&
+	>new &&
+	git add new &&
+	git commit -m new &&
+	git checkout -b test -t other &&
+	git reset --hard master &&
+	test_must_fail git pull
+'
+
+test_expect_success 'git pull non-fast-forward with merge (ff-only)' '
+	test_when_finished "git checkout master && git branch -D other test" &&
+	test_config pull.rebase ff-only &&
+	git checkout -b other master^ &&
+	>new &&
+	git add new &&
+	git commit -m new &&
+	git checkout -b test -t other &&
+	git reset --hard master &&
+	git pull --no-rebase
+'
+
 test_done
-- 
2.29.2